The present invention generally relates to ac devices, and particularly relates to synchronizing the operation of such devices to ac electrical systems.
Power measurement devices, such as electronic metering devices, are commonplace, and power conversion devices, such as dc-ac power inverters, are becoming more commonplace with the increased interest in alternative energy systems to provide localized power generation. Characteristically, these devices must synchronize themselves to an electrical system of interest. For example, electronic power meters synchronize their measurement operations with respect to the electrical system supplying the measured power. Similarly, localized alternative energy systems often are required to operate in grid-connected modes, wherein they are interconnected with an external electrical system, such as a utility company's power distribution grid.
Thus, there is a need for these ac electrical devices to lock with the phase and frequency of the electrical system. Approaches to phase locking in a three-phase electrical system typically exploit the three-phase nature of the electrical signals and synchronize a rotating reference frame to the three-phase system. Such approaches are not amenable to single-phase electrical systems. Zero crossing detection represents one approach to frequency detection and/or phase locking with respect to single phase electrical systems, wherein detection of the single-phase waveform's zero crossings is used to determine the system phase and frequency. Under nominal conditions, zero crossing detection provides a straightforward means of phase/frequency locking, but the approach suffers from its vulnerability to noise occurring at or around the zero crossing points.